How can I tell whether I need to change my study method rather than simply study longer?

If more study time produces little improvement in your understanding or test results, that is a sign to review your method rather than simply add hours. Try approaches that check what you can use independently—such as practice questions, recalling and explaining ideas, and analyzing mistakes—instead of only rereading, then compare your results over a short period. Also check sleep, breaks, and gaps in prerequisite knowledge.